Hi, to start I'm 15 years old and I'm on a pretty tight budget, $400. What I'm looking for is a stable computer that will be able to handle gaming, not at really high frame rates on the highest settings just something better than 20 fps at lowest settings across the board. Also it will be used for picture editing and video converting. But it doesn't have to be anything that crazy, since the best thing that I have even had is a stock ASUS CM1630-05.
Here what I have picked out, I looked at some off the build people are doing and picked certain things out.
This would get you a solid CPU with GPU so that you arent spending X dollars on a cheap GPU to just toss away later on. Then you could either try a Hybrid Crossfire utilizing the Radeon chip or a higher powered one. I selected a single 4GB chip since this smaller board only has 2 DIMMs, you could get 8GB (2x4GB) in a name brand chip for around $40 if you would like. There are tons of cases + PSU's that meet your budget, The HEC brand i've had good luck with in building 6 different mid end systems for Businesses and no duds. If you went with this build, you would have enough left over for a copy of windows 7 Home if you wanted to go that route as an alternative to Linux. Or if your set on the linux, your able to get a 1TB hdd or so and still come in under your $400